<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        li {
            list-style: none;
        }
        .box {
            width: 1228px;
            height: 375px;
            margin: 100px auto;
            background-color: #999999;
        }
        .box li {
            width: 292px;
            height: 375px;
            background-color: skyblue;
            float: left;
            margin-right: 20px;
        }
        /* 这里必须写 .box .last 要注意权重的问题 */
        .box .last {
            margin-right: 0;
        }
    </style>
</head>
<body>
        <ul class="box">
            <li>1</li>
            <li>2</li>
            <li>3</li>
            <li class="last">4</li>
        </ul>
</body>
</html>